Storage is how much space your files take up. This does not change as time passes unless you remove or upload files. Bandwidth is how much data our computers have sent to your audience on your behalf. This number typically increases steadily throughout the month, and is reset once per month.
In other words, think of storage like a storage locker, and bandwidth like monthly minutes on a cell phone plan.
The date your bandwidth will be reset is displayed on your My Account page. The reset will happen sometime during that day, United States Pacific time. If you need more storage or bandwidth, consider upgrading your membership.